Field of the Invention
[0001] The invention pertains to crossed-field amplifier tubes particularly tubes in which
the input signal is applied to a slow-wave interaction circuit which is part of the
cathode electrode.
Prior Art
[0002] In a crossed-field amplifier (CFA), a stream of electrons flows between an extended
cathode surface and a generally parallel anode surface. At least a part of the anode
is a wave propagating circuit with a wave velocity matched to the drift velocity of
the electron stream in the crossed electric and magnetic fields. In a non-reentrant
CFA, near the input end of the tube the electron stream is substantially confined
to a thin layer near the cathode surface. An input wave on the anode circuit has small
rf electric fields near the cathode, so the build-up of the trajectory modulation
to produce electrons striking the cathode with energy to produce secondary emission
multiplication is slow. Also the formation of charge spokes which induce waves in
the output part of the circuit is delayed.
[0003] It has been proposed in the prior art to make the cathode a second slow-wave interaction
circuit onto which the input signal would be applied. The rf fields of this circuit
would be high near the cathode surface and would interact strongly with the nearby
electron stream. Thus the build-up of current and wave energy could be much faster.
The charge spokes built up by the input cathode circuit would then drive the output
anode circuit. I have found that this prior-art scheme is often unsatisfactory. There
is inevitable coupling between the facing anode and cathode circuits. Also, the terminations
of the circuits at both their externally coupled input and output ends and at their
internal wave-absorbing terminations are never perfect, so that some of their waves
are reflected at the ends. Energy reflected from the output of the anode circuit can
be coupled back to form a backward wave in the cathode circuit. This backward wave
•can be partially re-reflected at the input of the cathode circuit. The resulting
forward wave is regenerative and can lead to instabilities. A second problem is that
waves on the anode circuit set up fields near the cathode circuit which can interfere
with the build up of space charge under control of the input circuit.
[0004] According to the invention there is provided a cross-field amplifier as set out in
claim 1 of the claims of this specification.
[0005] Examples of the prior art and of the invention will now be described with reference
to the accompanying drawings, in which:
FIG. 1 is a schematic cross-section of a prior-art CFA.
FIG. 2 is a schematic cross-section of a linear CFA embodying the invention.
FIG. 3 is a partial horizontal section of the CFA of FIG. 2.
[0006] FIG. 1 illustrates a prior-art CFA which comprises an input slow-wave circuit in
the cathode and an output slow-wave circuit in the anode.
[0007] The cathode structure 10 comprises a metallic block 12 as of OFHC copper, extended
in the direction of electron drift in the crossed D.C. electric and magnetic fields
in the open space 14 between cathode structure 12 and a parallel opposed anode structure
16. Anode structure 16 is typically operated at ground potential and forms a part
of the tube's vacuum envelope 18. Cathode structure 10 is operated at a negative potential
such that electron are drawn from it toward anode structure 16. A magnetic field is
applied perpendicular to the plane of FIG. 1 causing the electrons to drift toward
the right in the well-known crossed-field interaction.
[0008] Cathode block 12 is supported by one or more rods 20 mounted through dielectric insulating
cylinders 22 forming part of vacuum envelope 18. Rods 20 may be replaced by hollow
tubes (not shown) for carrying a fluid to cool cathode block 12.
[0009] A portion 24 of cathode structure 10, embedded in cathode block 12, is constructed
as a slow-wave circuit having a wave velocity comparable to the drift velocity of
electrons in the crossed fields. In FIG. 1 slow-wave circuit 24 is a meander line
formed by a meandering conductor 26 attached along its bottom side to cathode block
12 via ceramic supports 27. An input rf signal is coupled to the upstream end of slow
wave circuit 24 through a coaxial transmission line 28. The outer conductor 30 of
transmission line 28 is electrically integral with cathode block 12. The center conductor
32 passes inside outer conductor 30 and connects directly with the end of meandering
conductor 26 to introduce the rf drive signal. Coaxial line 28 is vacuum-sealed by
a transverse dielectric window 34 and is mounted on and insulated from vacuum envelope
18 by a high-voltage coaxial dielectric seal 36.
[0010] The initial electron stream for the amplifier comes from a thermionic cathode 38
mounted in a recess 40 in cathode block 12. It is heated by a radiant heater 42 which
is supplied with heating current via an insulating vacuum seal 44. This electron current
passes between the slow-wave circuits 24, 52 and is interacted on by the rf wave on
the cathode circuit 24 in such a way as to produce rf electron bunching and selective
bombardment of the cathode 10 by the electrons bunched in that phase of the rf wave
wherein they gain energy from the rf wave. Through the process of secondary emission
multiplication, the amount of bunched charge becomes greatly enhanced. The surface
46 of cathode structure 10 facing the electron stream may be coated with a material
having high secondary emission to increase the available electron current. At the
downstream end of slow-wave Circuit 24 a block of lossy dielectric 50 absorbs any
remaining wave energy in the circuit.
[0011] In the prior-art amplifier of FIG. 1 the high- level amplification and power extraction
is provided by a second slow-wave interaction circuit 52 embedded in anode block 16.
Output circuit 52 is similar to input circuit 24. Its input end is terminated by a
second lossy dielectric block 54. The downstream, output end of circuit 52 is directly
connected via a coaxial-to-waveguide transducer 56 to an output waveguide 58 sealed
by a dielectric vacuum window 60. The output power is carried by waveguide 58 to the
external useful load (not shown). Beyond the output end of slow-wave circuit 52 the
surface 62 of anode block 16 facing cathode block 12 is tapered closer to cathode
12. This causes collection of the spent electron stream over an extended area to spread
out the heat dissipation. Anode block 16 may be cooled by fluid coolant passages or
external air fins (not shown).
[0012] The purpose of the prior-art arrangement of FIG. 1 is to provide a crossed-field
amplifier with isolated input and output circuits and with high gain. In the original
crossed-field amplifier, the cathode was a smooth surface. The anode contained the
slow-wave circuit, connected to an input transmission line at one end and the output
transmission line at the other. Near the input, where the rf signal was small, the
electron stream is confined to a thin ribbon near the cathode by the transverse magnetic
field. The small rf electric field from the slow-wave circuit is a fringing field
from the main electric field between adjacent anode segments. It decays somewhat exponentially
with distance from the anode tips and is practically short-circuited by the conductive
smooth cathode. Thus the rf field interacting with the cathode-hugging electron stream
is small and the build-up of field and stream modulation is slow and the build-up
of charge through secondary emission multiplication is either slow, or for low power
levels, altogether non-existent. This limits the available gain of the tube. The gain
cannot be increased by merely lengthening the structure if the input power is insufficient
to trigger the charge build-up:
In the amplifier of FIG. 1 with the input signal on a slow-wave circuit which is part
of the cathode, the rf field of the circuit is much higher at the near-by electron
stream, so the interaction in the input region is much stronger than in a smooth-
cathode tube. When the electron stream becomes highly modulated into "spokes" and
loses some energy to the circuit, the "spokes" move close to the anode circuit and
instigate the output power therein.
[0013] The amplifier of FIG. 1 has however proven to be unsatisfactory due to regenerative
instability. One basic cause is that matches between slow-wave circuits and external
transmission lines are always imperfect. Particularly if a wide frequency range is
to be covered, there is some residual wave reflection at the junction. Also, mismatches
to the external signal generator and load create reflections. A large reflected wave
in the output circuit 52 can, by electromagnetic coupling, generate a small backward
wave in input circuit 24. This may be partially reflected at the input end of circuit
24 to produce a regenerated forward wave. Thus the spurious signal can build up until
oscillation is produced.
[0014] Another distorting process is interference by a large reverse directed wave on the
anode circuit with the build-up of charge through the secondary emission multiplication
process near the input. Such a large reverse directed wave can occur as a result of
reflections of the output wave from the load. Although the reverse directed wave is
non-synchronous with the forward directed electron flow at the input, its fields still
exert a significant influence on the relatively short electron trajectories in the
charge build-up region.
[0015] FIG. 2 illustrates a crossed-field amplifier tube embodying the invention. The parts
are structurally and functionally similar to those of FIG. 1, but the tube has a very
important distinction. The portion 64 of anode block 16 opposite cathode slow-wave
circuit 24 is smooth, so it does not carry waves at any velocity near that of the
electron stream or the input circuit 24. The output circuit 52 is removed downstream
of input circuit 24 and the surface 66 of cathode block 12 opposite anode circuit
52 is smooth.
[0016] The electron stream is modulated by cathode circuit 24. After passing beyond circuit
24 the stream carries the signal as a spatially modulated travelling charge pattern.
The beam charge induces an electromagnetic wave in anode circuit 52. This wave is
amplified by interaction of circuit 52 and the beam and coupled to the useful load
by output waveguide 58.
[0017] No wave, forward or backward, on anode circuit 52 can couple energy back to cathode
circuit 24 because they are spatially removed and because the electron stream moves
only from input to output and cannot carry retrograde modulation. With this inventive
improvement, the amplifier is made much more stable and greatly increased gain may
be obtained.
[0018] On the smooth portion of cathode block 12 before the end of anode circuit 52, part
68 of the surface facing anode block 16 may be formed of a material with low secondary
emission yield. Electrons returning to this surface will not be fully replenished
through secondary emission multiplication. As a consequence, charge will be drained
from the space between electrodes and only a reduced number of electrons will enter
the collector region to be collected on the high-potential anode where their bombarding
energy is much higher.
[0019] FIG. 3 is a partial section of the CFA of FIG. 2 taken on the horizontal plane 3. It
illustrates the form of the meander-line slow-wave circuit formed by the meandering
conductor 26. The input coupling via coaxial line 28 attached directly to the end
of conductor 26 is also shown.
[0020] The described embodiment of the invention is in a non-reentrant CFA. It is intended
to be illustrative and not limiting. Many other embodiments will become obvious to
those skilled in the art. The crossed-field amplifier may be in a circular form and/or
with a recirculating electron stream. In this case a long drift space free of rf waves
would follow the output circuit. It could incorporate irregular geometries to "scramble"
the electron stream to remove any residual modulation. The meander-line circuits may
be replaced by any of the other known slow-wave circuits, for example, coupled individual
vanes, helix coupled bars, stub-supported meander lines, etc. The degree of displacement
of the anode circuit beyond the end of the cathode circuit may vary, depending on
the particular tube design. For moderate gain tubes, the circuits may overlap to some
extent. The scope of the invention is to be limited only by the following claims and
their legal equivalents.
1. A crossed-field amplifier tube comprising:
an extended cathode (10),at least a section of an interaction surface of said cathode
being capable of emitting electrons, at least a section of said interaction surface
being part of a wave-transmissive cathode slow-wave circuit,
means for coupling an input transmission line(28)to- an input end of said cathode
slow-wave circuit,
an extended anode (16) having an interaction surface facing said cathode interaction
surface and spaced therefrom, at least a section of said anode interaction surface
being part of a wave-transmissive anode slow-wave circuit,
means for coupling an output end of said anode slow-wave circuit to an output transmission
line (58),
said cathode and anode being adapted for applying a DC electric field between said
anode and said cathode and a DC magnetic field parallel to said interaction surfaces,
said slow-wave circuits being adapted to transmit waves in the direction of drift
of electrons in said DC fields,
characterised in that said anode slow-wave circuit (52) is displaced from said cathode
slow-wave circuit (24) in said direction of drift.
2. The tube of claim 1 further comprising means for coupling a wave-absorbing load(50)
to the other end of said cathode slow-wave circuit.
3. The tube of claim 1 further comprising means for coupling a wave-absorbing load
to the other end of said anode slow-wave circuit.
4. The tube of claim 1 wherein said slow-wave circuits are displaced such that no
section of said anode circuit lies opposite said cathode circuit.
5. The tube of claim 1 wherein said cathode interaction surface comprises a section
not capable of propagating a slow wave, opposite at least a section of said anode
slow-wave circuit.
6. The tube of claim 5 wherein said non-propagating section extends opposite the full
length of said anode slow-wave circuit.
7. The tube of claim 1 wherein said emissive section comprises said slow-wave circuit
section.
8. The tube of claim 5 wherein said emissive section further- comprises said slow-wave
circuit section and at least a part of said non-propagating section.
9. The tube of claim 8 wherein said non-propagating section comprises a section of
low secondary electron emissivity displaced from said emissive section in said direction
of electron drift.
10. The tube of claim 1 wherein said cathode and said anode are separated by an electron-stream
passageway, said passageway extending in a closed loop for re-entry of an electron
stream, and wherein said cathode interaction surface and said anode interaction circuit
have opposed sections beyond the end of said anode interaction surface in said direction
of electron drift, on which waves do not propagate with velocities comparable to the
velocity of said drift.